The invention concerns a process for reducing the surface recombination velocity of silicon wafers.
When manufacturing electronic devices from silicon, a procedure for reducing surface recombination will usually be required for their functioning as well as for the application of various measurements and analytical methods. In particular, this is true for determining bulk recombination lifetimexe2x80x94which is a measure for the purity of silicon with regard to electrically active impurities. However, for the determination of this bulk recombination lifetime a necessary condition is that the smallest dimension of the sample must exceed four times the diffusion length of the free charge carriers. Generally, this requirement is not met by standard silicon wafers as their thickness is usually less than the diffusion length of the charge carriers. Only if the surface recombination velocity becomes sufficiently low, that is below 100 cm/s, will it be possible to measure bulk lifetime on standard silicon wafers with sufficient precision to monitor the purity of silicon wafers before and after technological processes for the device production.
A reduction of the surface recombination velocity in silicon can be achieved by thermally growing a silicon dioxide (SiO2) layer in an oxidizing atmosphere at temperatures in the range of about 800-1200xc2x0 C. In the literature, and in common usage, such layers are also frequently designated by the more comprehensive heading xe2x80x9csurface passivationxe2x80x9d. In general, surface passivation also includes the additional aspect of xe2x80x9cprotection against environmental influencesxe2x80x9d. Due to the excellent mechanical, chemical, and electronic properties of the SiO2 layer, today this type of layer is used in almost all areas of silicon semiconductor technology.
However, there are some applications where this type of layer cannot be used with satisfactory results. On the one hand, this is the case for devices which cannot withstand the high temperature-time stress occurring during the growth of this layer. Another case is the measurement of bulk lifetime in order to detect heavy metals in Si wafers in the ppb range. Here it is very difficult, or even impossible to quantify how oxidation will influence the heavy metal content of the wafer by additional contamination, or segregation, precipitation, and evaporation. On the one hand, depending on the ratio of contamination levels of wafer and oxidation furnace, contamination from the tube may diffuse into the wafer to be examined. But, on the other hand, it is also possible that contamination will outdiffuse from the wafer and accumulate in the oxide or precipitate homogeneously or heterogenously. In all these cases mentioned above, carrier lifetime measured subsequently no longer represents the original impurity level of the wafer. A further problem with thermal oxidation is the poor reproducibility of the density of states at the interface Si-SiO2, which determines the surface recombination velocity. FIG. 5 illustrates the lifetime distribution of a thermally oxidized wafer, with a low average lifetime of 85.84 xcexcs, which is mainly determined by the recombination at the Si-SiO2 interface.
To overcome the problem of thermal stress, there is the possibility of using a SiO2 layer deposited by the CVD (Chemical Vapor Deposition) process or one of its variantsxe2x80x94PECVD (Plasma Enhanced CVD) or Photo CVDxe2x80x94instead of the thermally grown oxide layer. Depending on the process used, temperatures from approximately 100xc2x0 C. to 900xc2x0 C. are applied. A further advantage of these processes is that even layers such as silicon nitride Si3N4 or silicon oxinitride SiOxNy can be deposited.
A serious disadvantage of these known deposited CVD layers is their poor ability to decrease the surface recombination velocity. Therefore they belong to the passivation layers in an extended sense, where the aspect xe2x80x9cprotection against environmental influencesxe2x80x9d takes precedence. For device applications, these layers will be used only in connection with a thin thermal SiO2 layer grown directly onto the silicon surface. For the same reason, and due to the fact that there are equipment specific contamination problems, so far no relevant analytical applications are known.
The analytical application is further complicated by the fact that the diffusion coefficients of the impurity metals of interest in Si are mostly some orders of magnitude higher than those of the doping elements P, B, As, and Sb. Therefore temperature-time stress, which is tolerable even for highly sensitive devices, can cause unpredictable changes in the contamination level, as already mentioned in the context of thermal oxidation.
Furthermore, it is known that hydrogen and halogens (F, Cl, Br, J, and At) present at the Si surface will reduce surface recombination velocity. This can be achieved, for instance, by treating the Si surface with hydrofluoric acid (HF). A serious disadvantage of this method is that the reduction of surface recombination disapears under the influence of atmospheric oxygen within a few minutes after removing the wafer from the liquid. This precludes any device application. A known application in the analytical area is the xe2x80x9cElymat Methodxe2x80x9d for measuring charge carrier diffusion length in Si wafers. Here, the wafer is placed inside a cuvette containing diluted HF during the measurement.
Due to the hazards involved in handling HF, a rather large technical expense is required to reduce risks for the operating personnel.
A method which has become known more recently is to put the Si surfaces, freshly etched with HF, into an alcoholic iodine or bromine solution, as described e.g. by H. Msaad, J. Michel, J. J. Lappe and L. C. Kimmerling in xe2x80x9cElectronic Passivation of Silicon Surfaces by Halogensxe2x80x9d (to be published in xe2x80x9cJournal of the Electrochemical Societyxe2x80x9d 1994). There, surface recombination velocities of less than 1 cm/s are achieved, which is an excellent value compared to about 100 cm/s achieved by thermal SiO2. A disadvantage of this methodxe2x80x94just as with the previous onexe2x80x94is that the effect remains stable only as long as the wafer remains in the solution. An advantage is the considerably lower hazard potential involved in handling an alcoholic iodine solution, compared to HF. Nevertheless, handling the liquid involves a substantial additional effort.
The object of the invention is to provide a simple-to-implement process enabling the surface recombination velocity of silicon to be reduced to values less or equal 100 cm/s, and which also allows easy handling of Si devices or Si wafers treated with this process.
According to the invention there are the following steps:
First, the Si surface will be cleaned as each Si surface usually exhibits a silicon dioxide layer (SiO2) approximately 2 to 4 nm in thickness. Preferably, this SiO2 layer can be removed by hydrofluoric acid (HF). After drying the Si surface, a lacquer will be applied to the surface of the Si wafer at a temperature of less than 100xc2x0 C., preferably at ambient temperature, so that when this lacquer dries an electrically non-conducting layer is formed. This lacquer may be applied, for example, by spraying, spinning, painting, or even by dipping.
Depending on the type of lacquer used, the drying process will lead to a consolidation of the lacquer as, for instance, a solvent present in the lacquer will evaporate, and optionally and additionally a chemical reaction with a reacting partner from the layer or ambient atmosphere (such as atmospheric oxygen, humid air) will cause a gel to form or a change in the physical condition to occur, or cooling will convert lacquer applied at a temperature above ambient temperature into a firm layer. For analytical use of the process according to this invention, i.e. to measure the carrier lifetime of standard Si wafers, the applied layer must be transparent for the laser beam used to carry out measurements, e.g. in the 900 nm range.
By this manner, surface recombination velocity will be reduced to less than 100 cm/s without a noticeable temperature stress occurring in the Si wafer or the Si semiconductor devices. This value is sufficiently small for the functioning of semiconductor devices as well as for the use of analytical methods such as the measurement of charge carrier lifetime in Si wafers to monitor contamination with heavy metals such as iron or gold.
Preferably, a material based on an organic substance may be used as lacquer, such as a natural or synthetic resin which will further contain a halogen as an additive, preferably iodine. Preferably, particularly good results were achieved by using iodine added to colophony, which is extracted from balsam resin, root resin, or tall resin. Equally good results are obtained by a lacquer based on the alkyd resin group of substances, such as alftalat. Furthermore, it has been shown that even a physically drying clear or transparent lacquer will give the desired results such as Zapon enamel which is based on cellulose nitrate.
Finally, using polysiloxans such as silicon lacquer will also yield useable results.